THE stress of opening a restaurant just before the economy swooned may have wrecked the marriage of Luke and Julie Janklow. He’s the literary agent once named “the hottest straight man in book publishing” and the nephew of Tavern on the Green creator Warner LeRoy. She opened Sweetiepie last fall, a family-friendly restaurant on Greenwich Avenue in which “lots of their friends invested,” said one source. And now they’re heading toward divorce. Real estate brokers are already eyeing the couple’s magnificent townhouse nearby.